Chemical Bonding in Crystalline
Compounds
Ulrich Wedig
Max-Planck-Institut für Festkörperforschung
Heisenbergstraße 1, D-70569 Stuttgart
The spatial arrangement of atoms in a crystalline solid can be
determined very precisely by modern analysis methods. It is, however,
rarely possible to deduce the driving forces for a certain arrangement
from the experimental data and to evolve relations to chemical concepts
like covalency, charge transfer and van der Waals interactions. In
metallic compounds a conceptual bonding analysis is even more complex,
as the subdivision of the entirety of valence electrons into localized
and delocalized parts is not obvious. On the other hand, the derivation
of bonding properties from quantum mechanical calculations comprises
certain arbitrariness. The bonding picture developed from the
theoretical data in any case has to be in conformance with the measured
properties.
The complex of problems is discussed by means of some examples:
(1) Platinides contain an anionic Pt substructure. The fraction of
itinerant electrons and the dimensionality of the anionic substructure
depend on the number of electrons provided by the cations.
(2) The structures of the group 12 elements deviate considerably from a
close packing. Electronic correlation with a significant involvement of
the filled d-shell is responsible for these deviations.
(3) Some crystal structures are difficult to understand on the basis of
standard electron counting rules. The assumption of localized electrons
in polyhedral voids of the cationic partial structure resolves these
difficulties.